Many inexpensive photographic cameras do not have the ability to adjust the lens focus based on the primary subject distance. The primary reason for excluding automatic focusing (AF) capability from cameras is the expense associated with conventional AF systems. A fixed-focus camera system can incorporate a less expensive lens and does not require the AF electronics and/or mechanical interfaces required in AF camera systems. It is well known, however, that a substantial system speed and image performance loss results from the exclusion of lens focusing capability in a fixed-focus camera system.
Attempts have been made to correlate ambient light levels with a preferred lens-focus position. These methods recognize that cameras without exposure control are likely to produce under exposed images below a threshold light level. An electronic flash unit must therefore be employed to obtain a useful film exposure. The range of an electronic flash unit is limited, however, thus the focus point for the camera lens must be brought within the flash range. If the flash is used and the ambient aperture and lens focus position are retained, unnecessary depth-of-field (extending well beyond the useful flash range) is imparted to the image, and flash power is wasted due to the small lens opening. If the ambient lens focus position is retained and a larger lens opening is used for flash, flash power is used more efficiently, but the focus point lies outside the optimum flash focus point. As a result, the depth-of-field is not correctly or optimally positioned within the flash range.
U.S. Pat. No. 4,710,013, for example, discloses a photographic camera system that controls lens position, flash and diaphragm/aperture settings based on a measured ambient light level. Although the disclosed system is described as an "autofocusing mechanism", the system can more appropriately be described as a light-level based lens positioning system, since the lens position is not related to the image location/subject position. Specifically, the lens is positioned alternately for distant and near objects based on the measured ambient light level. If the ambient light level is below a preset threshold level, the control system activates a flash unit, moves the diaphragm to a fully open position, and move the lens to a position for near objects. When the ambient light is above the preset threshold level, the control system deactivates the flash unit, sets the diaphragm at a reduced opening, and moves the lens to the distant position. Above the threshold light level, the largest aperture setting is the hyperfocal aperture corresponding to the far focus lens setting. A further reduction in aperture below the hyperfocal value is governed by a film speed entry system.
The system described above has a number of limitations. For example, while the system recognizes the futility in maintaining depth-of-focus sufficient to cover objects well outside the flash range, it is a practical fact that the difference between the hyperfocal aperture and the flash aperture decreases as the flash guide number increases. In such a case, the center of the flash exposure range (assuming a fixed flash output) is coincident with the daylight hyperfocal distance. As a result, the same aperture and lens focus position should be used for both flash and daylight scenes. Further opening of the aperture and reduction of the lens-focus point distance, however, would provide a poorer alignment of the depth-of-focus and the useful flash range which results in less satisfactory coverage of photospace. The system is therefore only useful for very low output flash units, and even then causes a serious tradeoff in photospace (light levels and distances) coverage in the flash mode. If the daylight nominal-daylight aperture were used, virtually all of the flash pictures would be underexposed which would be a more serious problem. Another potential problem results from the inability of the system to photograph scenes that may lie below the daylight set point, yet are beyond the flash range. Virtually all indoor or night sporting events, exhibitions and entertainment events fall into this category. One could incorporate a ranging autofocus system in conjunction with the exposure control system, but this would substantially increase the cost and complexity of the camera system.
In view of the above, it would be desirable to provide an autofocus system of simple and inexpensive design that provides improved photospace coverage, particularly for scenes that fall below daylight threshold levels, over the type of system described above.